1. Field of the Invention
The present invention relates to a device of predicting the life of a fuel cell, particularly a polymer electrolyte fuel cell for use in electric supply for portable apparatus, portable electric supply, electric supply for electric car, household cogeneration system, etc. and a fuel cell system.
2. Related Art of the Invention
A polymer electrolyte fuel cell causes the electrochemical reaction of a fuel gas such as hydrogen with an oxidizing agent gas such as air through a gas diffusion electrode to generate electricity. FIG. 13 is a schematic sectional view illustrating an ordinary configuration of such a related art polymer electrolyte fuel cell.
As shown in FIG. 13, the polymer electrolyte fuel cell 120 comprises a gas diffusion layer 101, a catalytic reaction layer 102, a polymer electrolyte membrane 103, a separator 104, a gas flow path 105, a cooling water flow path 107, an electrode 109, MEA 110, a gas sealing material 113, an O-ring 114, etc.
In some detail, the polymer electrolyte membrane 103, which selectively transports hydrogen ions, has a catalytic reaction layer 102 disposed in close contact with both sides thereof. The catalytic reaction layer 102 is mainly composed of a carbon powder having a platinum group metal catalyst supported thereon. The catalytic reaction layer 102 has a pair of gas diffusion layers 101 having both gas permeability and electrical conductivity, which are disposed in close contact with the respective external surface thereof. The gas diffusion layer 101 and the catalytic reaction layer 102 together form an electrode 109.
The electrode 109 has an electrode-electrolyte assembly (hereinafter referred to as “MEA”) 110 formed by the electrode 109 and the polymer electrolyte membrane 103 mechanically fixed to the outer side thereof. Adjacent MEA's 110 are electrically connected to each other in series. An electrically-conductive separator 104 having a gas flow path 105 on one side thereof through which a reactive gas is supplied into the electrode 109 and a gas produced by the reaction or extra gas is removed away and disposed of.
The gas flow path 105 may be provided separately from the separator 104 but is normally formed by providing a groove on the surface of the separator 104. On the other surface of the separator 104 is provided a cooling water flow path 107 through which cooling water for keeping the cell temperature constant is circulated. By circulating cooling water through the cooling water flow path 107, heat energy generated by the reaction can be used in the form of hot water or the like.
A gas sealing material 113 or O-ring 114 is provided on the periphery of the electrode 109 with the polymer electrolyte membrane 103 interposed therebetween to prevent hydrogen and air from leaking from the cell and being mixed with each other and prevent cooling water from leaking from the cell.
It is known that a fuel cell degradates with time when operated over an extended period of time. The degradated elements include the electrode catalyst, polymer electrolyte membrane, gas diffusion layer, etc.
As a method of previously detecting such degradation there has been proposed a method which comprises predicting the future drop of output voltage from the change of the output voltage of a fuel cell with time and hence the replacement time of the cell or stack (see, e.g., JP-A-1-122570). In accordance with this method, the degree of drop of output voltage due to excessive wetting of the electrode catalyst or electrolyte and the expected future output voltage are estimated on the basis of the pattern of change of the difference between the output voltage during ordinary operation and the output voltage under the conditions such that the oxygen concentration in the oxidizing agent gas is raised from the ordinary state with time to predict and judge the life of the cell.
Besides the above cited cell life predicting method, there has been proposed a method which comprises predicting the life of a fuel cell using an approximate equation for degradation rate of voltage and operating time with respect to basic operating pattern determined from the measurements of degradation rate of voltage of a fuel cell operated in basic pattern (see, e.g., JP-A-2002-305008). In accordance with this method, an approximate equation for degradation rate of voltage and operating time in basic operating pattern is used to calculate the ordinary voltage drop of the fuel cell and hence the life of the fuel cell.
Though being not a method of predicting the life of a fuel cell, as a method of prolonging the life of a fuel cell there has been a method which comprises increasing the electrolyte membrane thickness of apart of the electrode reaction portion of MEA as compared with other portions to inhibit local creep caused by clamping pressure (see, e.g., JP-A-11-97049). The polymer electrolyte membrane must be used in hydrous state to keep its protonic conductivity. Thus, the polymer electrolyte membrane can easily swell and undergo creep. This proposal is intended to increase the thickness of a part of the electrolyte membrane at the electrode reaction portion, which is remarkably moistened, so that the local drop of the thickness of the membrane due to compressive creep can be inhibited to prolong the life of the fuel cell.
However, the aforementioned method which comprises predicting the future voltage from the change of the output voltage of a fuel cell with time to predict the replacement time of cell or stack can have difficultly making sufficient prediction of suddenly occurring degradation because the life of the fuel cell is predicted from the change of the output voltage of the cell with time. Further, since the change of output voltage with time is judged from the difference between the output voltage during rated operation and the output voltage under the conditions such that the oxygen concentration in the oxidizing agent gas is raised from the ordinary state, it is necessary that the oxygen concentration in the oxidizing agent gas be raised once. Since it is usual to use air as an oxidizing agent gas except for special cases, an oxygen gas bomb must be always provided to raise the oxygen concentration. Moreover, this method is based on the assumption that it is applied to phosphoric acid type fuel cells but doesn't take into account the damage of electrolyte membrane which is likely to occur with polymer electrolyte fuel cells or the like.
The aforementioned method which comprises predicting the life of a fuel cell using an approximate equation for degradation rate of voltage and operating time with respect to basic operating pattern determined from the measurements of degradation rate of voltage of a fuel cell operated in basic pattern can have difficultly making sufficient prediction of sudden degradation behavior causing sudden voltage drop such as damage of polymer membrane similarly to the aforementioned method which comprises predicting the future voltage from the output voltage of a fuel cell.
The aforementioned method which comprises increasing the electrolyte membrane thickness of apart of the electrode reaction portion of MEA as compared with other portions to inhibit local creep caused by clamping pressure is likely to relax creep due to clamping of electrolyte membrane and hence prolong the life of a fuel cell. When the thickness of the polymer electrolyte membrane is increased, an effect of prolonging the life of the fuel cell can be exerted, but it doesn't mean that degradation no longer occurs. Accordingly, it is necessary that some method be used to predict the life of the fuel cell accurately and simply. This proposal has no reference to life prediction.
In other words, the related art method which comprises predicting the future voltage from the change of output voltage of a fuel cell with time to predict the replacement time of cell or stack as proposed in the above cited Patent Reference 1 is disadvantageous in that suddenly occurring degradation of a fuel cell cannot be sufficiently predicted.
The related art method is also disadvantageous in that an oxygen bomb must be always provided.
The related art method is further disadvantageous in that the damage of electrolyte which is likely to occur with polymer electrolyte fuel cells is not taken into account.
One objective of the present invention is to provide a fuel cell life predicting device for predicting the life of a fuel cell by properly judging the degradation of cell performance or the state of degradation of electrolyte membrane and a fuel cell system which addresses the aforementioned problems of the prior art methods.